1. Field of the Invention
The present invention relates generally to sphygmomanometers and, more particularly, to a pressure-dependent automatic leak valve for sphygmomanometers which is able to constantly reduce the pressure in a cuff without manipulation of a pressure reducing valve.
2. Description of the Related Art
Generally, intraarterial blood pressure periodically and repeatedly increases and decreases with systole and diastole, that is, according to the heartbeat. The maximum value in a periodic conversion cycle of blood pressure (BP) is called systolic pressure (PSYS), and the minimum value is called diastolic pressure (PDIAS). Furthermore, typically, blood pressure is expressed in PSYS/PDIAS, and its unit is mmHg.
The above-mentioned method of measuring blood pressure is noninvasive. Typically, a pressure obtained by pressing a portion of the arm of a human is expressed in Pc, and blood pressure in arteries in the arm is expressed in BP. Noninvasive blood pressure measurement uses a principle in that, because tissue, which is peripheral to the arteries, transfers compression pressure Pc to the arteries without loss, the arteries are closed or opened according to Pc and Bp.
If the compression pressure Pc is greater than the blood pressure BP, the arteries are closed. In the opposite case, the arteries are open. Therefore, if the Pc is sufficiently large, that is, in the case of Pc>PSYS, all arteries are closed. If the Pc which has been applied to the arm, is reduced, the arteries which have been closed, are slowly opened. Then, turbulent blood is forced through the narrowed arteries by the heart. At this time, it is possible to auscultate a sound around there. This sound is called a Korotkoff sound.
After this Korotkoff sound is heard, when the arteries are completely opened by further reducing the Pc, that is, when the state of Pc<PDIAS is reached, the sound disappears.
Here, the value of Pc at the point in time that the Korotkoff sound is heard is determined as a value of PSYS, and the value of Pc at the point in time that the Korotkoff sound disappears is determined as a value of PDIAS. Noninvasive blood pressure measurement is a method using this principle.
A mercury column sphygmomanometer is a representative example of sphygmomanometers using the principle of noninvasive blood pressure measurement. Typically, the conventional mercury column sphygmomanometer includes a cuff which is wound around the arm of a human to compress the arm using air injected into the cuff, and a mercury column which displays the pressure Pc in the cuff (the pressure is designated by the height of the mercury column and is expressed in mmHg units). The mercury column sphygmomanometer further includes an air injector which has a bulb shape and injects air into the cuff when a user squeezes the air injector with his/her hand, thus increasing the pressure in the cuff. The mercury column sphygmomanometer further includes a pressure reducing valve through which air is discharged. Here, the user can adjust a pressure reduction speed of the pressure reducing valve by rotating a screw type knob of the pressure reducing valve with the hand.
In the mercury column sphygmomanometer having the above-mentioned construction, as shown in FIG. 1, the air injector 10, having a bulb shape, is coupled at a first side thereof to the cuff 20 and communicates at a second side thereof with the atmosphere (A). First and second unidirectional valves 11 and 11′ are provided in the air injector 10 at the first and second sides at which the air injector 10 communicates with the cuff 20 and the atmosphere (A), respectively. The pressure reducing valve 30 is disposed around the first unidirectional valve 11 which is located at the side related to the cuff 10. If the user pushes the air injector 10 in an inside direction of the arrow (M) of FIG. 1 while the pressure reducing valve 30 is closed, the second unidirectional valve 11′, which is located towards the atmosphere (A), is closed and the first unidirectional valve 11, which is located towards the cuff 10, is opened by the manual pressure generated in the air injector 10. Thus, air, which has been in the air injector 10, flows in the direction of the arrow of FIG. 1 and is injected into the cuff 20. When the user releases the air injector 10, the air injector 10 is returned to the initial state by its elasticity, that is, the air injector 10 returns in the outward direction of the arrow (M) of FIG. 1. Then, the first unidirectional valve 11 is closed and the second unidirectional valve 11′ is opened by negative pressure occurring in the air injector 10. Thus, outside air is drawn into the air injector 10 through the second unidirectional valve 11′. The pressure in the cuff 20 is increased by repeatedly conducting this process.
To reduce the pressure in the cuff 20 for measuring blood pressure, the pressure reducing valve 30 is open, so that air in the cuff 20 is discharged outside. Then, the pressure in the cuff 20 is reduced. At this time, the user is able to regulate the speed of pressure reduction in the cuff 20 using the screw structure of the pressure reducing valve 30. However, this regulation is very sensitive, so that a high level of skill has been required.
The operation of measuring blood pressure using such a mercury column sphygmomanometer will be explained herein below. As shown in FIG. 2, the cuff 20 is wound around an arm. Thereafter, while the pressure reducing valve 30 is completely closed, air is injected into the cuff 20 by the air injector 10 until the pressure Pc in the cuff 20 becomes sufficiently higher than the systolic pressure PSYS. Then, the pressure of a mercury column gauge 40 is increased. After air is injected into the cuff 20 such that Pc becomes sufficiently higher than PSYS, the pressure reducing valve 30 is slightly opened so as to slowly reduce Pc. Typically, Pc is reduced at a speed ranging from −3 to −5 mmHg/sec. While Pc is gradually reduced, the Korotkoff sound occurs. When the Korotkoff sound is heard, the user immediately reads the height of the mercury column using a stethoscope 50 and determines it as the systolic pressure PSYS. Continuously, while the Pc is reduced, the user immediately reads the height of the mercury column at the point of time that the Korotkoff sound disappears, and determines it to be the diastolic pressure PDIAS.
However, to measure blood pressure using the conventional mercury column sphygmomanometer, sufficient skill is required. In other words, the user must precisely hear the occurrence and elimination points of the Korotkoff sound with the ears and, simultaneously, must precisely reads the height of the mercury column at those points with the eyes. As well, the user must carefully regulate the pressure reducing valve such that the pressure reduction speed becomes −3 to −5 mmHg/sec.
Meanwhile, if a computer is used, the computer controls a motor of the pressure reducing valve, but, because there is a probability of malfunction of the motor, problems still remain.